The present invention relates to a thin film element and a method for manufacturing the thin film element, and more particularly to a tunnel-type Josephson thin film element which can be applied to high frequency communication and a logic circuit and utilizes the intrinsic Josephson junction of an oxide high-temperature superconductor, and a method for manufacturing the tunnel-type Josephson thin film element.
Recently, thin film elements having various functions have been made for practical use with the development of the thin film forming technique. As thin film elements using oxide thin films, an infrared ray sensor which utilizes pyroelectric properties of a lead titanate based thin film, a surface acoustic wave filter which utilizes piezoelectric properties of a zinc oxide thin film, and the like have been made for practical use, for example. Referring to these thin film elements, the standard of crystallization of the thin film which is required to become an element is not high. In addition, since the thin film element has a simple crystalline structure, it can easily be formed into a thin film having the crystalline properties which are necessary. For these reasons, the thin film element has been made for practical use without lots of difficulty.
In an element which utilizes the great anisotropy of crystals, however, the characteristics of the element is highly influenced by the disturbance of small crystalline properties (the orientation of crystals). Consequently, a thin film having great crystalline properties is required. For example, when a very thin insulating layer is interposed between two superconductors, the tunnel effect of an electron pair is generated between the superconductors. Consequently, even though a voltage is not applied from the outside, a superconductive current flows through the thin insulating layer. Thus, a tunnel-type Josephson element is obtained. Actually, it has been hard to manufacture the tunnel-type Josephson element in which a thin oxide insulating layer is interposed between oxide high-temperature superconductors because the coherence length of a superconductive electron pair is very small.
Recently, it has become apparent that the two-dimensional crystalline structure of the oxide high-temperature superconductor essentially forms a Josephson junction, that is, an intrinsic Josephson junction. Japanese Unexamined Patent Publication No. 7-58366 has also proposed a method for manufacturing a tunnel-type Josephson element as a thin film element which is made of the oxide high-temperature superconductor.
A method for manufacturing a tunnel-type Josephson element according to the prior art will be described below with reference to the drawings.
FIGS. 13 (a) to 13 (e) are sectional views showing, in order of steps, the method for manufacturing a tunnel-type Josephson element according to the prior art. The method for manufacturing a tunnel-type Josephson element will briefly be described sequentially. First of all, a first superconductor thin film 102 having a thickness of about 100 nm is formed on a substrate 101 and a photoresist pattern 103 is then formed selectively on the upper face of the first superconductor thin film 102 as shown in FIG. 13 (a).
By using the photoresist pattern 103 as a mask, the first superconductor thin film 102 is etched, for example, by 10 nm as shown in FIG. 13 (b).
As shown in FIG. 13 (c), a layer insulating film 104 is deposited over the whole face of the substrate 101 to the same extent as the amount of etching.
As shown in FIG. 13 (d), the photoresist pattern 103 is lifted off to remove the layer insulating film 104 on the photoresist pattern 103. Then, the surface covered with the photoresist pattern 103 on the first superconductor thin film 102 is cleaned by oxygen ion beams. Thereafter, a second superconductor thin film 105 having a thickness of about 150 nm is deposited over the whole face of the substrate 101 with a vacuum kept. Subsequently, the second superconductor thin film 105 is etched so as to have a width of 100 .mu.m as shown in FIG. 13 (e).
In the method for manufacturing a tunnel-type Josephson element according to the prior art, however, the crystalline properties of a junction region 102a which is an interface between the first superconductor thin film 102 and the second superconductor thin film 105 that is shown in FIG. 13 (e) become poorer. More specifically, the layer structure of a crystal is not held over the junction region 102a but becomes partially discontinuous at the atomic level as shown in a plan view of FIG. 14 (b) which is an enlarged view showing the junction region 102a in FIG. 13 (e) and in FIG. 14 (a) showing a sectional structure taken along the line XIV--XIV in FIG. 14 (b). In some cases, consequently, electric insulation is not sufficiently performed by a high resistive layer (block layer) in the layer structure of the crystal. Therefore, top and bottom low resistive layers (in the c-axis direction) are electrically connected in the layer structure of the crystal so that tunnel junction characteristics cannot be obtained.
The junction region 102a shown in FIG. 13 (d) is formed by the fine processing technique. For this reason, when forming the first superconductor thin film 102, it is necessary to obtain a single crystal region having a size which can be processed by at least the current fine processing technique (about 1 .mu.m or more) so as to be easily distinguished from other regions. In the current thin film forming technique, however, it is hard to obtain the single crystal region which meets such a condition.
Furthermore, the steps of forming a first thin film, performing fine processing, and forming a second thin film are necessary when forming the junction region 102a. Therefore, impurities such as water molecules easily adhere to the junction region 102a, for example. Accordingly, when the impurities adhere to a c-axis orientation film in the oxide high-temperature superconductor having an intrinsic Josephson junction in which a superconductive layer and an insulating layer are alternately oriented, two or more insulating layers are newly laminated on the insulating layer so that the tunnel junction characteristics cannot be obtained. Thus, when completely forming the superconductor thin film by performing processing plural times, it is difficult to obtain a junction of good quality.
According to the method proposed in the Japanese Unexamined Patent Publication No. 7-58366, thus, it is impossible to manufacture a tunnel-type Josephson thin film element having substantially excellent element characteristics.